1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device produced by mounting semiconductor elements on a printed circuit board.
2. Description of Prior Art
In recent years, the trends of application for semiconductors with a relatively large number of pins, such as a MPU, have been in the direction of a narrower pitch arrangement of electrodes, two-dimension lattice array of electrodes, or front arrangement of pin electrodes. Semiconductor devices with highly densified multi-pins are therefore coming on so that the electrodes can be formed close to or over the functional portions of elements. For conventional methods for mounting semiconductor elements on a printed circuit board using a flip chip technique, a method is known in which solder bumps are formed on the electrode faces of the semiconductor elements, as described in Japanese Patent Publication JP-A 53-87596. These solder bumps, which are composed of a solder alloy, are connected to the electrode pads on a circuit board by soldering. There are also known methods in which gold-stud bumps are formed on the electrode faces of a semiconductor and these bumps are connected to substrate electrode pads using a conductive adhesive (see Japanese Patent Publication JP-A 62-140264) or using a solder alloy (see Japanese Patent Publication JP-A 6-213615).
In these technologies of forming the bumps on the electrodes of a semiconductor, the substrate on which the semiconductors are mounted, requires the same flatness as a semiconductor chip. Proposal has been made of a method of forming the bump electrodes on a substrate side but the chip (see Japanese Patent Publication JP-A 4-19766). In general, ceramic substrates do not have such a flatness in its surface as semiconductor chips. This imparts the problem that a stable connection to electrode pads of the semiconductor cannot be made since all the tips of the bumps have no coplanarity even if the bumps with the same height are formed on the surface of the substrate.
Furthermore, there are methods in which an anisotropic conductive film is used to bind a semiconductor element to a substrate by hot pressing. In this bonding method, dielectric breakdown in the anisotropic conductive film is caused only at the position between the bump electrode and the electrode pad to electrically connect both of them. In this bonding method, the tips of the bump electrodes on the side of the substrate are also uneven. As there is a difference in intervals between each of the bump electrodes embedded into the adhesive film and the corresponding electrode pad of the semiconductor element, the bump electrodes having a wider gap between their tips and the corresponding electrode pads are not allowed to make electrical contact with the corresponding pads even if the semiconductor element is pressed against the substrate. It is uncertain to secure the electrical contact between all bump electrodes and the corresponding electrode pads.